In general, ultra-sound imaging techniques use a pulse-echo method wherein short pulses of ultrasonic energy generated by a piezoelectric transducer are focused to a narrow beam and transmitted through a suitable conducting medium, usually water, into the body of a patient. A portion of the ultrasonic energy is reflected back toward the transducer at tissue interfaces between various different bodily structures due to mechanical impedance discontinuities at the interfaces. The transducer receives the reflected energy and converts it into electrical signals. The time of arrival of the returning reflected signals indicates the relative positions within the body of the interfaces. In other words, the timed spacing between the reflected signals or echoes is proportional to the physical spacing of the respective reflecting interfaces within the body; and the amplitude of the echo is a function of the characteristics of the structures forming the interface.
The image-representing electrical signals corresponding to the characteristics of the reflected mechanical energy are then displayed, generally, on a "B-scan" display. Such a display is comparable to a conventional television display. In such a system, the reflected echo signals modulate the brightness of the display at each point scanned. Strongly reflecting internal structures, such as hardened artery walls, appear brighter on the display than weakly reflecting structures. This gray scale produces a useful diagnostic tool.
A plurality of scan lines can be produced by scanning the ultrasonic beam produced by the transducer at a predetermined rate and in a predetermined direction across the surface of the patient. The plurality of scan lines so produced can be used to yield a sector-shaped display of a cross-sectional picture in the plane of the scan produced by the reflector-scanner, which scans mechanically over a desired angle.
A typical sector scan probe is illustrated in U.S. Pat. No. 4,151,834, the disclosure of which is hereby incorporated by reference. This patent shows a housing containing a DC servomotor that drives a mechanism for oscillating a transducer crystal. The transducer is periodically pulsed with electrical signals causing an ultrasonic beam to be emitted periodically as the transducer oscillates between its angular limits. The rate at which the transducer is pulsed is many times greater than the rate of angular movement of the transducer; for this reason, the beam is said to scan a sector.
Some medical procedures are facilitated when a biopsy needle is used in conjunction with the ultrasonic probe, and for this reason, spatial ultrasonic probes have been developed to achieve this purpose. U.S. Pat. No. 4,108,165 discloses an ultrasonic probe having an annular transducer periodically driven by an electronic circuit for producing ultrasonic beams which can be directed into the body of a patient under examination. In the device disclosed in this patent, no scanning of the beam is provided, and the transducer is mounted in a cylindrical housing having an axial bore concentric with the annular transducer. The biopsy needle is concentrically located within the probe to align the biopsy needle with a biopsy target in the body under examination.
In general, high accuracy biopsy can be accomplished using a biopsy attachment mounted on a scanning ultrasound probe where the attachment is of the type which includes a needle guide for orienting the biopsy needle accurately toward a biopsy target according to the following procedure. First, the patient is ultrasonically scanned and the biopsy target is located on the display screen. The display also includes a predetermined, superimposed electronic representation of the position of the needle-line relative to the probe scan head, where the needle-line is defined as the line along which the biopsy needle would travel while being inserted through the needle-guide. To align the needle-line with the biopsy target, the ultrasound probe is moved relative to the body under examination until the displayed needle-line passes through the image of the biopsy target on the display screen. In order to determine the distance to the biopsy target, a movable cursor associated with the display device can be moved to the displayed biopsy target. By calibrating the position of the cursor, the distance to the target can be determined.
The above-described technique is generally illustrated by U.S. Pat. No. 4,346,717 which discloses an ultrasonic probe designed to facilitate the use of a biopsy needle. This system produces and electronically superimposes on a display screen image of the body under examination a guide image beam which corresponds to the orientation of a needle guide for a puncturing biopsy needle. The coordinates of the guide image beam on the display screen are calculated by using the value of the angle .theta. which defines the angular relationship between the needle guide and the ultrasonic probe. An angle detector is employed for precisely detecting the angle of the guide sleeve relative to the ultrasonic probe. If the guide sleeve is arranged at a fixed angle .theta., aiming of the puncturing needle relative to the target area is accomplished by spatially displacing the ultrasonic probe on the surface of the body, and adjusting the attitude of the probe, until the guide beam superimposed on the displayed image passes through the biopsy target area to be punctured. If the position of the guide sleeve is adjustable with respect to the probe, adjusting the guide sleeve causes the guide image beam to likewise be adjusted via positioning signals which are obtained as a function of an angle-adjusting element which functions to reposition the guide sleeve; thus, the guide sleeve is moved until the guide image beam passes through the biopsy target shown on the display screen.
Both the above-described general ultrasound biopsy procedure as well as that disclosed in U.S. Pat. No. 4,346,717 require the generation of a guide image beam which electronically represents the biopsy needle-line on the display screen and which is superimposed on the displayed ultrasound image of the scanned body section which includes the biopsy target. In heretofore known systems, in order to show the needle-line on the display, the system must determine, for example, by direct measurement or precision manufacturing, the geometry of the needle guide, and hence that of the needle-line, in the scan head coordinate system which defines the sector sweep of the ultrasound scanning head associated with the ultrasound imaging apparatus; this coordinate system can conveniently be expressed in polar coordinates employing the scan angle and the range r.
However, in most ultrasound probe systems having biopsy attachments, although the spatial relationship between the needle guide and the housing is known because the biopsy attachment is mounted on the probe housing, the spatial relationship between the scan head and the probe housing is generally not precisely defined because the imaging system does not require it. As a result, the geometry of the needle guide in the scan head coordinate system is also not precisely defined. The system disclosed in U.S. Pat. No. 4,346,717 solves this problem by providing a needle guide attachment which is, unlike most ultrasound probe systems, precisely disposed at a known angle .theta. with respect to the ultrasound scan head. Disadvantageously, this requires a high degree of manufacturing precision with respect to the tolerances between the biopsy attachment, the probe housing, and the scan head, resulting in increased costs. It also requires calibration at the factory. Alternatively, as noted above, the system disclosed in U.S. Pat. No. 4,346,717 employs an angle detector to determine this angle, also resulting in additional components and increased costs.
It would be advantageous to eliminate the necessity of determining the particular spatial relationship between the scan head and the needle-line in order to calibrate the biopsy attachment with the scan head coodinate system so that the needle-line can be superimposed on the display screen. It is, therefore, an object of the present invention to provide an ultrasonic imaging apparatus biopsy attachment calibration method and apparatus which provides this capability and which overcomes the above-described deficiencies in the prior art.